Nuclear power

Nuclear power obtained from the fission of uranium and plutonium nuclei represents a significant percentage of world energy resources. Its production in appropriately designed nuclear fission reactors is especially important as a low-pollution supplement to fossil fuels.

Nuclear Fission

The idea of the atom as a source of energy developed near the beginning of the twentieth century after Antoine-Henri Becquerel discovered radioactivity. The energy of this spontaneous emission, first measured by Pierre and Marie Curie, was found to be far greater than ordinary chemical energies. Nuclear fission was discovered in 1939 after Otto Hahn and Fritz Strassmann had bombarded uranium with neutrons at their laboratory in Berlin, leaving traces of radioactive barium. Their former colleague Lise Meitner and her nephew Otto Frisch calculated the enormous energy—about 200 million electron volts—that would be released in reactions of this type. These results were reported to Niels Bohr and quickly verified in several laboratories in 1939. Soon Bohr developed a theory of fission showing that the rare isotope uranium-235 (uranium with 235 nucleons: 92 protons and 143 neutrons) is far more likely to produce fission, especially with slow neutrons, than the common isotope uranium-238, which makes up 99.3 percent of natural uranium. It also was recognized that if a sufficient number of neutrons were emitted in fission, they could produce new fissions with even more neutrons, resulting in a self-sustaining chain reaction. In this process, the fissioning of one gram of uranium-235 would release energy equivalent to burning about three tons of coal.

The first nuclear reactor to achieve a controlled, self-sustaining chain reaction was developed under the leadership of the Italian physicist Enrico Fermi in 1942 at the University of Chicago. To increase the probability of fission in natural uranium, only 0.7 percent of which is uranium-235, and to prevent any chance of explosion, the neutrons were slowed down by collisions with carbon atoms in a graphite “moderator.” It was necessary to assemble a large enough lattice of graphite (385 tons) and uranium (40 tons) to achieve a “critical mass” of fissile material, in which the number of neutrons not escaping from the “pile” would be sufficient to sustain a chain reaction. Cadmium “control rods” were inserted to absorb neutrons during construction so that the chain reaction would not begin the instant the critical size was reached.

The uranium-235 isotope is the only natural material that can be used to produce nuclear energy directly. By early 1941, however, it was known that uranium-238 captures fast neutrons to produce the new element plutonium. Plutonium has a 24,000-year half-life and is fissile, so it can be used as a nuclear fuel. Plutonium can be “bred” in a uranium reactor from uranium-238 with excess neutrons from the fissioning of uranium-235. One other fissionable isotope, uranium-233, can be obtained by neutron capture from the thorium isotope thorium-232. Uranium-233 is a possible future nuclear fuel. Uranium from which uranium-235 has been removed is almost entirely uranium-238 and is called “depleted uranium.” Depleted uranium has few uses, mostly exploiting its low cost but high specific gravity of 19 (almost twice the density of lead). It is used where massive weights are needed in small spaces, like counterweights for aircraft control surfaces and for armor-piercing artillery shells.

Thermal Reactors

The two basic types of reactors in use are thermal reactors, which use slow neutrons, and fast breeder reactors, which use fast neutrons to breed plutonium. Plutonium can be separated by chemical methods, but very expensive physical methods are necessary to separate uranium-235 from uranium-238; these methods involve many stages of gaseous diffusion or centrifuge processes to distinguish between their slightly different masses. Most nuclear reactors use 3.5 percent enriched uranium, but there are also reactor designs that use natural uranium. Weapons-grade uranium is usually enriched to 93.5 percent. Fast breeder reactor fuel is generally enriched with plutonium.

Thermal reactors for generating useful power consist of a core that contains a critical assembly of fissionable fuel elements surrounded by a moderator to slow the neutrons, a coolant to transfer heat, and movable control rods to absorb neutrons and establish the desired fission rate. Reactor fuel elements are made of natural or enriched uranium metal or oxide in the form of thin rods clad with a corrosion-resistant alloy of magnesium, zirconium, or stainless steel. Moderator materials must be low neutron absorbers, with small atomic masses close to the mass of neutrons so that they can slow them down by repeated collisions. Most reactors use moderators made of graphite, water, or heavy water, which contains the hydrogen isotope deuterium. Ordinary water is low in cost and doubles as a coolant, but it absorbs neutrons about one hundred times more than graphite and about one thousand times more than heavy water. Coolants such as water, carbon dioxide, and helium transfer heat liberated by fission from the core, producing steam or hot gas to drive a turbine for generating electricity in a conventional manner. Control rods are made of high neutron absorbers, such as cadmium or boron, and can be adjusted for any desired power output.

Light Versus Heavy Water Reactors

In 2017, reactors in the United States were light water reactors (LWRs): they used ordinary water as both moderator and coolant and required some fuel enrichment. The World Nuclear Association reported that as of April 2017, approximately 65 percent of the LWRs were pressurized water reactors (PWRs), while the remaining LWRs were boiling water reactors (BWRs). Most LWRs use 2 to 3 percent enriched uranium dioxide fuel elements clad in a zirconium alloy, although the PWR was first developed with much higher fuel enrichments for compact shipboard use. In a PWR, water is circulated at high pressure through the reactor core at above 300 degrees Celsius and then through a heat exchanger, where steam is produced in a secondary loop. In a BWR, the water is boiled in the core at about 280 degrees Celsius, eliminating the high cost of an external heat exchanger and highly pressurized containment vessel. LWRs have the fail-safe feature: If the temperature increases fast enough to expel water from the core, neutrons will be slowed less effectively, and the fission rate will decrease.

Canada has specialized in heavy water reactors since Canada has access to natural uranium with no need for fuel enrichment. In the Canadian deuterium-uranium system (CANDU), the heavy water coolant is circulated past fuel elements inside pressure tubes, which are surrounded by a heavy water moderator in a low-pressure tank. The coolant is pumped through a heat exchanger to boil ordinary water for driving steam turbines. Since 1968, several CANDU plants in the 200 to 700-megawatt range have been built in Canada, Argentina, India, Pakistan, and South Korea. Variants of this system employ light water or gas as a coolant to reduce the high cost of heavy water, but they may require enriched fuel.

Fast Breeder Reactors

The main alternatives to thermal reactors are fast breeder reactors, which can obtain about fifty times as much energy from natural uranium by producing more plutonium from uranium-238 than the uranium-235 they use. Because neutron capture by uranium-238 requires fast neutrons (about one thousand times faster than thermal neutrons), no moderator can be used, and a 15 to 30 percent fuel enrichment is needed to sustain the chain reaction. A typical breeder core consists of a compact assembly of fuel rods with 20 percent plutonium and 80 percent depleted uranium (most uranium-235 is removed) oxides surrounded by a “blanket” of depleted uranium carbide to absorb neutrons and yield more plutonium. The “liquid metal fast breeder reactor” (LMFBR) uses sodium in liquid form (above 99 degrees Celsius) as a coolant since water would slow the neutrons. Loss or interruption of sodium can lead to a meltdown of the core, so some designs seal the core in a pool of sodium.

The first commercial fast breeder reactor began operating in the Soviet Union in 1972, producing 350 megawatts. France had the most advanced fast breeder reactor program, with its 1,200-megawatt Super Phénix breeder reactor. India has successfully converted thorium into uranium-233 and used it as fuel in the ICGAR fast breeder test reactor. In its pure form, thorium is a silver-white metal similar to uranium. Thorium metal is more stable in air, retaining its luster for months, whereas uranium quickly tarnishes. Since thorium is three to four times more abundant than uranium, the ability to use it in commercial reactors would greatly extend the nuclear fuel supply.

Nuclear Fusion

Most of the problems associated with fission power could be eliminated with nuclear fusion reactors. These problems include the handling, storage, and reprocessing of highly radioactive materials such as plutonium; the possible theft of such materials by terrorists; the disposal of radioactive waste products; the dangers of a reactor accident; and the limited availability of fission fuels. The fusion of hydrogen isotopes to produce helium releases energy comparable to fission but requires no critical mass of fuel that might cause a meltdown. It has many fewer radioactive products with no storage or disposal problems and uses a fuel of almost unlimited supply. About 0.01 percent of the hydrogen in ocean water is in the form of deuterium. To overcome electrical repulsion and bring deuterium atoms close enough to cause a fusion reaction, an ignition temperature of about 100 million degrees is required. Ignition and isolation of such reactions require some kind of magnetic confinement of a plasma (ionized gas) or inertial confinement of deuterium pellets. Energy would be extracted by nuclear reactions in a surrounding lithium blanket caused by neutrons emitted during fusion. Some progress has been made in achieving these requirements, but a practical source of fusion power is many years away.

Experiments at several laboratories in the 1980s claimed to have found evidence for room-temperature fusion by using electrolysis to draw deuterium ions into the crystal lattice of hydrogen-absorbing materials such as palladium or titanium. Because electrical repulsion between charged nuclei increases greatly as they approach each other, it is difficult to understand how this process could bring deuterium ions close enough for fusion to occur. Even if such experiments were confirmed and explained, the development of a reactor to produce electrical power with this technique may be difficult, if not impossible. Hydrogen absorption declines sharply with increasing temperature, decreasing by a factor of at least ten as the temperature approaches the boiling point of water. Much higher temperatures would have to be produced for an efficient steam-driven electrical generator. Most physicists believe the initial results were the result of poor experimental design.

Safety of Nuclear Power

The study of reactor safety involves estimating the biological effects of radiation and analyzing the risk factors in possible reactor accidents. Information on the effects of large doses of radiation is based on medical X-rays, animal experiments, and studies of Japanese atomic-bomb survivors. Radiation doses are monitored by photographic film dosimeters and simple ionization chambers. Normal background radiation from radioactivity in the earth, radon gas, and cosmic radiation is about double the average dose received by a person for medical purposes annually. The radioactivity from normal reactor operation is considerably less than background radiation. The major public concern focuses on accidental releases of large amounts of radioactivity. The Nuclear Regulatory Commission estimates the risk from a reactor accident at less than one death over its service lifetime.

The risks of a nuclear accident can be studied only when one such accident occurs. By 2024, the most serious commercial reactor accident in the United States occurred in 1979 at the Three Mile Island power station in Pennsylvania; the loss of some coolant led to the shutdown of one reactor, as designed, but resulted in costly damage to the core. Because of containment structures, including a thick steel vessel around the core and a reinforced concrete building with walls several feet thick, the highest average dose released was about one-tenth the annual background radiation. A much more serious accident occurred at Chernobyl in the Soviet Union in 1986; a loss of coolant in a graphite reactor led to increased power, followed by explosions and fire, killing thirty-one men. Approximately 270,000 people were evacuated from areas in the former Soviet Union, where the average radiation from Chernobyl fallout ranged from 6 to 60 millisieverts (mSv). While these levels are above the average of 2.2 millisieverts for natural background radiation, they are no higher than levels that occur naturally in some regions of Brazil, India, and China. In 2011, a large earthquake in Japan resulted in serious damage and radiation leaks at the nuclear plant at Fukushima. This disaster was eventually given a hazard rating equal to that of Chernobyl. According to the National Research Council report on the health effects of low levels of ionizing radiation, although an increased frequency of chromosome aberrations is found, no increase in the frequency of cancer has been documented in populations residing in areas of high natural background radiation.

The disposal of radioactive waste is another area of concern and continuing study. Methods of solidifying such waste in glass or other materials for confinement in metal canisters and burial are being studied. The solid waste projected through the year 2010 would cover about 40,000 square meters (10 acres). Of the several disposal sites and methods under study, the most likely is deep underground burial in formations of salt or rock. The principal problem is plutonium-239, which has a half-life of 24,000 years, meaning waste would have to remain isolated for a million years or more. Eventual reprocessing of the waste in breeder reactors might alleviate this problem.

Future of Nuclear Power

Nuclear power is an important source of low-pollution energy, despite serious problems that have emerged since 1980. According to the International Atomic Energy Agency (IAEA), in 2016, 19.7 percent of electrical energy in the United States was generated by one hundred nuclear reactors capable of producing 100,351 megawatts of power. Approximately 2,476 terawatt-hours of electrical energy worldwide were generated by 451 nuclear reactors. The Nuclear Energy Agency reported in 2016 that using 56,600 metric tons of uranium as of 2015, the reactors produced more than 375,000 megawatts of power annually. At that rate, known and unproved uranium reserves were anticipated to last about another 135 years. Reprocessing spent fuel and using the military’s excess of highly enriched uranium (HEU) may extend the supply for many years, and new reserves are still being discovered. The use of fast breeder reactors rather than thermal reactors would extend nuclear fuel supplies by a factor of fifty or sixty, while the use of thorium could add an additional factor of three or four. According to the U.S. Energy Information Administration, in 2023, approximately 18.2 percent of US electricity generation came from nuclear energy.

The future of conventional nuclear power is uncertain. Although there have only been three significant nuclear power plant accidents in the fifty-year history of nuclear generation (Fukushima, Chernobyl, and Three Mile Island), public distrust of nuclear power has increased. The concern about reactor safety has led to new requirements that have raised the cost of nuclear power plants by a factor of five above inflation. Bankers and investors have become increasingly cautious about financing new plant construction. The high rate of government subsidies has complicated the evaluation of real dollar costs of nuclear power, but estimates indicate that the profit potential of conventional nuclear power is about half that of coal power. The total US nuclear power capacity, operational and planned, slipped from 236 reactors in 1976 to 104 in 1999 and 94 in 2024. By March 2024, however, the US produced about 30 percent of the world's nuclear electricity and was the largest producer of nuclear power worldwide.

Concerns about climate change have pushed the need for more energy created from nuclear power. In many countries, the creation of nuclear power plants became part of their plans for transitioning to green energy to fulfill their pledges to prevent the earth from heating more than 1.5 degrees Celsius from preindustrial levels.

One promising approach to the safer production of nuclear fission power is the development of small-scale modular reactors that use tiny ceramic-coated fuel pellets in small enough quantities in their cores that meltdown is impossible. Although initially expensive, such units would not require expensive safety systems and could be built on an assembly line, producing one module at a time to match operating capacity with the demand for power. Using advanced technology, China, Russia, India, the United Arab Emirates, South Korea, and other nations have been expanding their use of nuclear energy. According to the IAEA's 2021 report, seventy SWRs reactors were under construction worldwide. The first SMR to begin commercial operation was Russia's Akademik Lomonosov in May 2020.

At the December 2023 United Nations Climate Change Conference (COP28), 198 member nations called for accelerating the use of nuclear power and other low-emission energy sources to achieve the rapid and deep decarbonization necessary to meet global heating limits. More than 20 member nations set a goal of tripling their nuclear power capacity to reach net-zero emissions by 2050. By the end of 2023, 413 reactors in 31 nations were providing a global operational nuclear power capacity of 371.5 gigawatts electrical.

Principal Terms

half-life: the time required for half of the atoms in a given amount of a radioactive isotope to disintegrate

isotopes: an element’s variant forms, whose atoms have the same number of protons but different numbers of neutrons

moderator: a material used in a nuclear reactor for slowing neutrons to increase their probability of causing fission

millisieverts (mSv): a measure of the biological effect of radiation; 1,000 mSv in a short time will result in radiation sickness, and 4,000 mSv will kill half of those exposed

nuclear fission: the splitting of an atomic nucleus into two lighter nuclei, resulting in the release of neutrons and some of the binding energy that held the nucleus together

nuclear fusion: the collision and combining of two nuclei to form a single nucleus with less mass than the original nuclei, with a release of energy equivalent to the mass reduction

radioactivity: the spontaneous emission from unstable atomic nuclei of alpha particles (helium nuclei), beta particles (electrons), and gamma rays (electromagnetic radiation)

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